1. Field of the Invention
The present invention relates to a polarization converter for converting a light beam from a light source into a single linearly polarization beam, an illumination optical device having the polarization converter and a projector.
2. Description of Related Art
Conventionally, a projector has been used for presentation at a conference, academic society and exhibition. Such projector accommodates a plurality of optical components in a casing, where a light bean irradiated by a light source is enlarged and projected after being modulated by the light components to form a projection image.
A polarization converter for converting the light beam from the light source into a single linearly polarization beam is used for enhancing utilization efficiency of the light beam to obtain a bright projection image.
The polarization converter has a polarizing conversion element having a polarization separating film inclined relative to incident light beam for separating the light beam from the light source into two linearly polarization beam (separates into two linearly polarization beam by transmitting and reflecting the light beam), a reflecting film for reflecting one of the separated linearly polarization beams (linearly polarization beam reflected by the polarization separating film) separated by the polarization separating film and a light-transmissive member interposed between the polarization separating so that a plurality of the polarization separating films and the reflecting films are alternately arranged, and a retardation plate attached to light-irradiation side of the polarizing conversion element.
Such polarization converter has an ineffective area, where the light beam is directly incident on the reflecting film without passing through the polarization separating film to be irradiated as ineffective polarization light.
Especially, since the light beam from the light source has the highest luminance at the central position (illumination optical axis), which is gradually lowered in accordance with the distance from the central position, considering such characteristics of the light source, the polarization converter has a pair of polarizing conversion elements spaced apart with each other at a predetermined gap and located so that the corresponding polarization separating films and the reflecting films are opposed In other words, the par of polarizing conversion elements are located spaced apart with a predetermined gap sandwiching the illumination optical axis of the light source.
According to the above arrangement of the polarization converter, the light beam passing through the gap between the pair of polarizing conversion elements (i.e. through the central position) is a random polarization light including effective polarization light, thus improving utilization efficiency of the light beam.
In the polarization converter having the pair of polarizing conversion elements, the respective polarization separating films are inclined from opposing sides of the pair of polarizing conversion elements approximately in reverse V-shaped cross section and the reflecting films are disposed in parallel to the inclined polarization separating film with a predetermined interval, the polarization separating films and the reflecting films being alternately and consecutively arranged. The retardation plate is attached to an end of the light-irradiation side of the polarizing conversion element in accordance with the pitch of the polarization separating film.
Recently, high luminance has come to be required for a projector and further improvement in light utilization efficiency is desired.
The luminance of the projector may be enhanced by reducing the gap between the pair of polarizing conversion elements of the polarization converter and converting the random polarization light passing through the gap between the pair of polarizing conversion elements (illumination optical axis) into a single effective polarization light
However, according to the above polarization converter, the retardation plates attached to opposing sides of the pair of polarizing conversion elements interferes with each other when the gap between the pair of polarizing conversion elements is diminished.
In order to avoid the interference, the attachment accuracy of the retardation plate has to be more strictly set, thus complicating the production process of the polarization converter.